CVE-2026-99031: net: bridge: avoid recursive multicast port cleanup

In the Linux kernel, the following vulnerability has been resolved:

net: bridge: avoid recursive multicast port cleanup

brmulticastsgdelexcludeports() removes automatically added STAREXCL port groups by calling brmulticastdelpg(). For S,G entries, that deletion calls back into brmulticastsgdelexcludeports(), so a large number of STAREXCL ports consumes one kernel stack frame per port and can hit the stack guard page.

Keep the complete port-group deletion path for cleanup, but suppress the S,G exclude-port cleanup while that helper is already walking the same entry. Normal deletion callers retain the existing behavior.